As shown in FIG. 13, there is a control device that drives an AC motor 80 with a DC power supply 82 via a three-phase PWM inverter main circuit 81. Such a control device generates PWM waveforms Ea, Eb and Ec by level comparison between sine wave signals Va, Vb and Vc of each phase and a carrier wave signal C of a triangular wave as shown in FIG. 14. Then, by applying the PWM waveforms, each switching element of the three-phase PWM inverter main circuit 81 is controlled in switching to obtain a sine wave output. In FIG. 14, reference characters Vab and Van refer to a line voltage and a phase voltage, respectively.
A sine wave level and a phase are controlled to control an output current and a phase supplied from the three-phase PWM inverter main circuit 81 to a load such as an induction motor IM (AC motor) 80 or a permanent magnet motor (AC motor) not shown in FIG. 13. In order to detect a current used for the control, in vector control, an output current at three points P1 to P3 connecting the main circuit and the load is detected for three phases (or two phases) by a current sensor using a Hall CT.
However, a general purpose inverter intended to reduce cost is demanded to reduce the number of current sensors. Thus, there is a method of providing only one current sensor on a DC side (point P4 in FIG. 13) of a PWM inverter, and calculating an output current from the DC current. However, in this method, a voltage E of a DC power supply 82 of the PWM inverter is substantially constant, and thus an average value of detected DC currents changes in proportional to an output frequency of the inverter. Thus, the average value of the DC currents at low frequency is small, which reduces current detection accuracy to make it difficult to obtain high performance.
To address these problems, for example, Patent Document 1 discloses an output current detection device of a PWM inverter that uses the fact that current information (−Iw and Iu) of two phases: a minimum phase (Vw*) and a maximum phase (Vu*) of a voltage command value appear twice within a PWM carrier wave period in a DC input current IDC of a PWM inverter, samples a DC input current IDC based on PWM pulse signals, distributes the DC input current IDC for each phase, and detects three-phase current detection values.
However, in a current detection method disclosed in Patent Document 1, for example, as shown in FIG. 15, when an intermediate phase Vv of three-phase voltage command values Vu, Vv and Vw (corresponding to Va, Vb and Vc in FIG. 14) is close to a maximum phase Vu or a minimum phase Vw, or when an output voltage level is low, rising edges of a three-phase PWM signal are close to each other to reduce a pulse width, thereby making current detection impossible. Thus, Patent Document 2 discloses a control device and a control method for an AC motor and a module that automatically reduces carrier wave frequency of an inverter to increase a pulse width in the above-described case, and facilitates current detection.
However, in the method shown in Patent Document 2, problems such as generation of noise or reduced efficiency that occurs when the entire carrier wave frequency is reduced can be solved, but the carrier wave frequency is reduced only when the intermediate phase of the voltage command values is close to the maximum or minimum phase, or when the output voltage level is low. Thus, the carrier wave frequency is reduced by software, which is considerably difficult.
Thus, the applicant has already proposed a control device for an AC motor as described below (see Patent Document 3).
Specifically, there is proposed a control device for an AC motor that includes DC input current measuring means to a three-phase PWM inverter that drives the AC motor by inputting DC, and estimates a current to the AC motor from a measurement result of the DC input current measuring means to control the AC motor, including: an AC motor current detection unit that is connected to the DC input current measuring means and calculates each of three-phase currents in the AC motor; a three-phase voltage command calculation unit that calculates a three-phase voltage command provided to the AC motor from the three-phase currents calculated by the AC motor current detection unit; and a three-phase PWM waveform creation unit that has a carrier wave generation unit that generates carrier waves having a certain interval, and generates a three-phase PWM inverter control signal by level comparison between three-phase voltage reference signals of a U phase, a V phase and a W phase that constitute the three-phase voltage command calculated by the three-phase voltage command calculation unit and the carrier waves generated by the carrier wave generation unit, wherein the three-phase PWM inverter control signal is generated by level comparison between the three-phase voltage reference signals and the carrier waves.
As such, the carrier waves having a certain interval are generated by the carrier wave generation unit, and the three-phase voltage reference signals of the U phase, the V phase and the W phase calculated by the three-phase voltage command calculation unit are compared with the carrier waves generated by the carrier wave generation unit. Thus, for example, even when an intermediate phase of voltage command values is close to a maximum or minimum value or when an output voltage level is low, a plurality of, for example, two or three carrier waves have a certain interval. This prevents rising ends of a three-phase PWM signal from being close to each other to reduce a pulse width, prevents current detection from being made impossible, and allows current detection with high accuracy using an easy and simple configuration.